The president of the Laborers' International union refused to answer questions yesterday at a hearing by the President's Commission on Organized Crime, during which a local president was linked to an attempt on the life of an organized crime boss.

Angelo Fosco, president of the 625,000-member union for the past 10 years, was asked to explain why 156 federal indictments were returned against members of his union from 1980 to 1984. He declined to respond, invoking the Fifth Amendment.

Fosco invoked his right to avoid self-incrimination six more times, refusing to respond to questions about organized crime members in the union, organized crime influence in union locals and even whether he was aware that AFL-CIO ethics discourage reliance on the Fifth Amendment.

Earlier, Robert Powell, a retired international vice president, testified he was threatened when he expressed interest in running against Fosco in 1981. Powell said he took the threats seriously. He moved his family out of Washington, D.C., and started carrying a gun and wearing a bullet-proof vest, he said.

The union's general counsel, Robert Connerton, did not appear before the 13-member panel hearing three days of testimony on labor racketeering, and commission lawyers obtained a court order directing him to testify by 1 p.m. today, said the panel's spokesman, Arthur Brill.

The panel also refused to delay an appearance, scheduled today, by Teamsters President Jackie Presser. Presser's attorney asked for the delay, citing a recommendation from the Justice Department's Cleveland Strike Force that Presser be indicted. The strike force recommended charges of ghost employee payments.

"Given the foregoing, I have advised Mr. Presser that at this point in time, he must exercise his Fifth Amendment rights before the commission," his attorney, John R. Climaco, said in a letter to the panel.

A former Chicago gambling boss, Ken Eto, who is in the government's witness protection program, told the panel a 1983 attempt on his life was ordered by Vincent Solano, president of Laborers' union Local 1 in Chicago.

Solano was one of three Local 1 officers who invoked the Fifth Amendment. Solano has never been charged with a crime in connection with the attempt on Eto's life.

Frank DeMonte, identified as an officer in the local, also invoked the Fifth Amendment. Salvatore Gruttadauro, also identified as a Local 1 officer, responded to questions with a statement challenging the constitutionality of the commission's inquiry, contending it grew out of illegal wiretaps.

Eto, wearing a black hood and robe and flanked by US marshals, testified that on the night of Feb. 10, 1983, he met two men, Jasper Campise and John Gattuso, who were supposed to take him to meet Salano for dinner.

Instead, he testified, they drove to a parking lot and "as soon as I parked - bang. I got shot in the head." Eto was twice more, but his assailants were off target and Eto played dead.

Campise and Gattuso were charged with attempted murder. Their bodies were found in a car trunk in July 1983. They had been stabbed many times, and the authorities have said they suspect the two were killed for failing to kill Eto.

Eto told the commission he had pleaded guilty earlier to a federal gambling charge and met with Solano, to whom he had been reporting for nearly a decade, a few weeks before the shooting to talk about his options. Solano said Eto could serve time, appeal or flee, according to Eto.

Commissioner Thomas McBride said law enforcers believe organized crime exerts substantial control over the 1.4 million-member Teamsters union, the Laborers union, the 400,000-member Hotel and Restaurant Employees union and the 200,000-member International Longshoremen's Association.

"Membership of the 'big four' internationals in large part embraces some of the poorest, hardest-working, most under privileged members of our society," he said.

". . . In too many cases their unions have become a vicious and vital part of the machinery of exploitation."